The invention relates to a method and an apparatus for determining a shape of an optical test surface.
The shape of high-precision optical free form surfaces, the deviation of which from rotational symmetry clearly exceeds the dynamics of a conventional interferometer, is measured using a so-called compensation system (C system). Such a compensation system, also referred to in the following as adaptation optics, is configured to form from an incoming wave having, for example, a plane or spherical wavefront, a wave the wavefront of which is identical to the desired shape of the free form surface to be measured. In many cases a computer-generated hologram (CGH) or a combination of a number of CGHs is used as adaptation optics.
In the case where adaptation optics consisting of two CGHs are used, partial calibration of the adaptation optics using a spherical calibrating mirror is possible as described in US 2009/0128829 A1. Unknown deviations remain however.
Rotationally symmetrical spheres are measured with great precision using rotation averaging. Rotation averaging is understood as meaning the recording and processing of a series of measurements each with different rotational positions of the specimen. The rotation averaging enables one to separate asymmetrical specimen and system errors of the adaptation optics. Rotationally symmetrical errors can not be separated and must be assessed by a complex theoretical budget consideration. The rotation averaging brings about, furthermore, averaging of shortwave errors which arise due to small deviations in the interferometer beam path as a result of non-perfect interferometer components. With free form surfaces it is not possible to implement the rotation averaging due to the lack of rotational symmetry. Due to this both the rotationally symmetrical and the asymmetrical errors of the adaptation optics remain undetermined and must be assessed by budget consideration. Furthermore, the averaging of shortwave errors produced in the interferometer is dispensed with.